Prior to being fed to an extruder, hygroscopic plastic resin granules or pellets (particles) typically are dried to remove deleterious moisture. Heretofore, drying of the plastic resin particles has been accomplished by either an open loop, hot air drying system or a closed loop, desiccant drying system.
The open loop, hot air drying system includes a conical drying hopper in which the plastic resin particles to be dried are accommodated and a heater compartment/blower unit for supplying heated air to the hopper for dispersal over the resin particles by an air dispersal mechanism in the hopper. Typically, ambient air is drawn into the blower, heated to the required drying temperature (usually 160.degree. to 300.degree. F.) in the heater compartment, and discharged into a lower region of the hopper where it is dispersed to flow upwardly around the resin particles. The particles are thereby heated and release moisture which is drawn from the top of the hopper for discharge into the surrounding atmosphere.
The open loop, hot air drying system has inherent limitations as a result of the use of the heated ambient air to effect the drying action. In particular, the heated air itself may not be sufficiently dry to remove enough moisture from the plastic resin particles to enable extrusion of acceptable end products, particularly under humid ambient conditions that are prevalent in certain periods of the year (e.g., the summer months) and/or in certain geographic regions. This limitation is especially acute with respect to the extrusion of moisture-sensitive plastic resins, such as nylons and ABS.
The closed loop, desiccant drying system currently is the most widely used for drying plastic resin particles in preparation for extrusion. This system employs a conical drying hopper containing the resin particles to be dried and an in-line desiccant dryer. In this system, moisture-laden air from the top of the hopper is recycled in to back to the desiccant dryer to remove the moisture. The dried air is then passed through blower and heater compartments and returned to the lower region of the hopper. Closed loop, desiccant drying systems typically operate using -40.degree. F. to dewpoint airflow through the plastic resin particles.
The closed loop, desiccant drying system is advantageous in that it is virtually unaffected by the moisture conditions of the ambient air. However, the desiccant dryer must be sized for the air flow of the drying hopper to achieve proper operation thereof and the air flow through the desiccant material must be maintained at an appropriate velocity over the desiccant material for proper moisture removal. The typical rule of thumb for plastic resin particulate drying requires an airflow of one cubic foot per minute per pound of plastic resin particles to be dried per hour (1 cfm/lb/hr). Unfortunately, this rule dictates use of a large, expensive desiccant dryer unit that is sized for the air flow requirements, rather than for the moisture removal capacity of the desiccant material. Thus, the closed loop, desiccant drying system has high equipment costs associated therewith.
An object of the present invention is to provide an improved closed loop apparatus and method for drying moisture-bearing particles wherein a first airflow loop is used for heating and removing moisture from the particles in a container, a second desiccant drying loop is used for removing moisture from a portion of the moisture-laden return airflow, and an airflow valve is provided for adjusting as needed the relative proportions of moisture-laden return airflow and dried return airflow in the heated airflow supplied to the container for effective drying of the resin particles.
Another object of the invention is to provide an improved closed loop apparatus and method for drying moisture-bearing particles wherein, by virtue of use of the dual closed loops in conjunction with the airflow valve, the desiccant dryer can be sized to the moisture removal capacity of the desiccant material so as to permit use of a smaller desiccant dryer unit and thereby provide a significant reduction in equipment cost.